Gradient Fining Tank for Refining Foamy Molten Glass and a Method of Using the Same

ABSTRACT

A gradient fining tank and a method of operating the tank to refine foamy molten glass is disclosed. The gradient fining tank includes a floor, a roof, and two laterally-spaced sidewalls that at least partially define an interior chamber of the tank. The floor of the tank is profiled to provide the tank with an extended shallow portion that defines an inlet to the interior chamber and a deep holding portion that defines an outlet from the interior chamber. An entry section of the floor provides the extended shallow portion of the tank and a transition section and exit section of the floor provide the deep holding portion. A depth of the interior chamber at an outlet end of the deep holding portion is greater than a depth of the interior chamber at the outlet end of the extended shallow portion.

The present disclosure is directed to a gradient fining tank configuredto refine foamy molten glass and, more specifically, to a gradientfining tank that includes an extended shallow portion and a deep holdingportion downstream of the extended shallow portion in a flow directionthrough interior tank chamber.

BACKGROUND

Glass is a rigid amorphous solid that has numerous applications.Soda-lime-silica glass, for example, is used extensively to manufacturea variety of hollow glass articles including containers such as bottlesand jars. Soda-lime-silica glass comprises a disordered and spatiallycrosslinked ternary oxide network of Na₂O—CaO—SiO₂. The silica component(SiO₂) is the largest oxide by weight and constitutes the primarynetwork forming material of soda-lime-glass. The Na₂O componentfunctions as a fluxing agent that reduces the melting, softening, andglass transition temperatures of the glass, as compared to pure silicaglass, and the CaO component functions as a stabilizer that improvescertain physical and chemical properties of the glass including itshardness and chemical resistance. The inclusion of Na₂O and CaO in thechemistry of soda-lime-silica glass renders the commercial manufactureof glass containers more practical and less energy intensive while stillyielding acceptable glass properties. Soda-lime-silica glass, in generaland based on the total weight of the glass, includes 60 wt % to 80 wt %SiO₂, 8 wt % to 18 wt % Na₂O, and 5 wt % to 15 wt % CaO.

In addition to SiO₂, Na₂O, and CaO, soda-lime-silica glass may, ifdesired, include other oxide and non-oxide materials that act as networkformers, network modifiers, colorants, decolorants, redox agents, orother agents that affect the properties the final glass. Some examplesof these additional materials include aluminum oxide (Al₂O₃), magnesiumoxide (MgO), potassium oxide (K₂O), carbon, nitrates, fluorines,chlorines, and/or elemental or oxide forms of one or more of iron,arsenic, antimony, selenium, chromium, barium, manganese, cobalt,nickel, sulfur, vanadium, titanium, lead, copper, niobium, molybdenum,lithium, silver, strontium, cadmium, indium, tin, gold, cerium,praseodymium, neodymium, europium, gadolinium, erbium, and uranium.Aluminum oxide is one of the more commonly included materials—typicallypresent in an amount up to 2 wt % based on the total weight of theglass—because of its ability to improve the chemical durability of theglass and to reduce the likelihood of devitrification. Regardless ofwhat other oxide and/or non-oxide materials are present in thesoda-lime-glass besides SiO₂, Na₂O, and CaO, the sum total of thoseadditional materials is preferably 10 wt % or less, or more narrowly 5wt % or less, based on the total weight of the soda-lime-silica glass.

Soda-lime-silica glass for containers and other end-uses has long beenproduced in a continuous melting furnace in which a glass feedstock isfed onto a large molten glass bath-on the order of hundreds of metrictons—of generally constant level contained in a melting chamber of acontinuous melting furnace. The molten glass bath is maintained at atemperature of about 1450° C. or greater so that the added glassfeedstock can melt, react, and progress through several intermediatemelt phases before becoming chemically integrated into the molten glassbath as the bath moves through the melting chamber of the furnacetowards a refining chamber located on the opposite side of a submergedthroat. In the refining chamber, bubbles and other gaseous inclusionsare removed from the glass to yield chemically homogenized and refinedmolten glass having the correct chemistry as needed for furtherprocessing. For instance, in the manufacture of glass containers, therefined molten glass is transported from the refining chamber, through aforehearth, and into to a glass feeder where precise gobs of moltenglass are sheared and dropped into a gob delivery system. The gobs ofglass are delivered to a blank mold and formed into a parison or apartially-shaped hollow container. The parison is then transferred to ablow mold and enlarged into the final container.

The heat needed to maintain the molten glass bath within the meltingchamber has conventionally been supplied by burners that combust a fueland oxidant mixture within an open combustion zone located above themolten glass bath. The burners are located in burner ports on oppositesidewalls of the refractory superstructure that partially defines thecombustion zone (cross fired furnace) or in a back wall of therefractory superstructure (end port fired furnace) and typically combusta mixture of natural gas and air or pure oxygen (oxy fuel). To improvethe energy efficiency of the burners if air is employed as the oxidant,the flue gases created by the combustion process are used to preheat theair fed to the burners through recuperative or regenerativeheat-recovery techniques. Electrical boosting may also be used tointroduce additional heat into the molten glass bath by passing anelectric current through the bath between submerged electrodes in anattempt to enhance the temperature uniformity within the bath andimprove furnace output.

The operation of conventional continuous melting furnaces for glassproduction is very energy intensive given the large quantity of moltenglass held within the melting chamber of the furnace and the relativelylong residence times, usually ranging from 18 hours to 24 hours.Continuous melting furnaces are operated indefinitely as long as theyare structurally and functionally operational in order to achieve alevel of energy efficiency that supports the economics of glass productmanufacturing and also to preserve the structural integrity of thefurnace structure. Shutting down a conventional continuous meltingfurnace and emptying the molten glass bath is not economicallypractical, even when the production of glass end-products is halted foran extended period of time, and risks cracking the refractory materialsused to construct the furnace due to thermal shock. Rather, during aproduction stoppage, the addition of glass feedstock into the furnace issuspended, the removal of molten glass from the furnace is halted, andthe molten glass bath contained in the melting chamber is sustained inits current state through the requisite addition of heat from theburners. Steady-state operation of the furnace may then be resumed byremoving refined molten glass as needed to meet production needs andfeeding glass feedstock into the molten glass bath to offset the amountof refined molten glass being removed.

Submerged combustion (SC) melting is a melting technology that hasrecently become a potentially viable alternative to the glass feedstockmelting process used in a conventional continuous melting furnace.Contrary to conventional melting practices, SC melting involves firingthe fuel and oxidant mixture directly into and under the surface of amolten glass bath contained in a melter, typically though submergedoxy-fuel burners mounted in the floor or sidewalls of the melter. Thefuel and oxidant mixture autoignites and the resultant combustion gassescause vigorous stirring as they jet through the molten glass bath. Thiscreates intense heat transfer and rapid particle dissolution throughoutthe bath and contributes to a reduction in the residence time of moltenglass within the melter compared to a melting chamber of a conventionalmelting furnace. Indeed, in a SC melter, the residence time of moltenglass may be as little as three to four hours. To that end, the SCmelter can be smaller than the melting chamber of a continuous meltingfurnace and still provide a comparable output of molten glass. Thesmaller size of the SC melter, and the fact that it can be externallycooled, enables it to be shut down and emptied, and then restarted,quickly and efficiently when necessitated by production schedules orother considerations.

While SC technology can melt the glass feedstock to produce chemicallyand thermally homogenized molten glass relatively quickly, the moltenglass bath contained within the SC melter has a tendency to be foamy asa result of the strong turbulence generated in the bath by operation ofthe submerged burners. In particular, the forced delivery of combustiongasses into the molten glass bath contributes to widespread gas bubbles,seeds, and/or dissolved gases within the molten glass bath. Existingfining chambers utilized in connection with conventional continuousmelting furnaces may not be well-adapted for refining the low-densityfoamy molten glass removed from the SC melter in a time-efficientmanner. One particular challenge that is unique to SC melting technologyis that a stable layer of bubbles tends to form at the upper surface ofthe molten glass, which, in turn, blocks the further ascension andescape of bubbles contained in the glass while also inhibiting heattransfer into the underlying portion of the glass. A fining tank that isdesigned to refine foamy molten glass, particularly in a plant setting,would help support the successful implementation of SC meltingtechnology in the overall manufacture of glass products including, mostnotably, the manufacture of glass containers.

SUMMARY OF THE DISCLOSURE

The present disclosure describes a gradient fining tank and a method ofusing the gradient fining tank to refine foamy molten glass receivedfrom, for example, a glass melting furnace such as an upstream submergedcombustion melter. The gradient fining tank includes a floor, a roof,and two laterally-spaced sidewalls that at least partially define aninterior chamber of the tank. The floor of the tank is profiled toprovide the tank with an extended shallow portion that defines an inletto the interior chamber and a deep holding portion that defines anoutlet from the interior chamber. Relative to a flow direction thoughthe tank interior chamber, the floor includes an entry section, atransition section, and an exit section. The transition section and theexit section constitute the portion of the floor included in the deepholding portion, which is disposed downstream from the extended shallowportion, and a depth of the interior chamber at the outlet end of thedeep holding portion is greater than a depth of the interior chamber atthe outlet end of the extended shallow portion. Additionally, thetransition section of the floor has a flow surface that slopesdownwardly and outwardly from a flow surface of the entry section of thefloor in the extended shallow portion of the tank.

The present disclosure embodies a number of aspects that can beimplemented separately from or in combination with each other to providea gradient fining tank particularly adapted to refine foamy molten glassreceived from a submerged combustion melter. According to one embodimentof the present disclosure, a method of refining foamy molten glassincludes several steps. In one step, foamy molten glass is introducedinto an interior chamber of a gradient fining tank defined at least inpart by a roof, a floor, and a pair of laterally-spaced sidewalls thatconnect the roof and the floor. The foamy molten glass is introducedinto the interior chamber through an inlet of the gradient fining tankthat communicates with an upstream portion of the interior chamberdefined by an extended shallow portion of the tank. Upon introductioninto the upstream portion of the interior chamber, the foamy moltenglass intermixes with a molten glass bath that is maintained within theinterior chamber of the gradient fining tank.

In another step, the molten glass bath is directed to flow along theextended shallow portion of the gradient fining tank from an inlet endof the extended shallow portion to an outlet end of the extended shallowportion. The extended shallow portion includes an entry section of thefloor, and the molten glass bath separates into a higher-density phaseand a lower-density phase that rests on top of the higher-density phasewithin the extended shallow portion of the gradient fining tank. In yetanother step, the molten glass bath is directed to flow into adownstream portion of the interior chamber defined by a deep holdingportion of the gradient fining tank. The deep holding portion has aninlet end, which communicates with the outlet end of the extendedshallow portion, and an outlet end. The deep holding portion furtherincludes a transition section of the floor that extends downwardly andoutwardly from the entry section of the floor of the extended shallowportion to an exit section of the floor. A depth of the downstreamportion of the interior chamber at the outlet end of the deep holdingportion is greater than a depth of the upstream portion of the interiorchamber at the outlet end of the extended shallow portion.

In another step, the molten glass bath is directed to flow along thedeep holding portion from the inlet end of the deep holding portion tothe outlet end of the deep holding portion. The higher-density phase ofthe molten glass bath sinks and collects as refined molten glass withinthe downstream portion of the interior chamber above the exit section ofthe floor at the outlet end of the deep holding portion of the tank.And, in still another step, refined molten glass is removed from theinterior chamber of the gradient fining tank through an outlet of thegradient fining tank that communicates with the downstream portion ofthe interior chamber. The outlet of the gradient fining tank isdownwardly displaced from the inlet of the gradient fining tank.

According to another aspect of the present disclosure, a method ofrefining foamy molten glass includes several steps. In one step, agradient fining tank is provided that includes a roof, a floor, and apair of sidewalls that connect the roof and floor. The roof, floor, andsidewalls at least partially define an interior chamber that holds amolten glass bath. The gradient fining tank has an extended shallowportion that defines an upstream portion of the interior chamber and adeep holding portion that defines a downstream portion of the interiorchamber. Moreover, the extended shallow portion of the tank includes anentry section of the floor and the downstream portion of the tankincludes a transition section of the floor that extends downwardly andoutwardly from the entry section of the floor to an exit section of thefloor such that a depth of the downstream portion of the interiorchamber at an outlet end of the deep holding portion is greater than adepth of the upstream portion of the interior chamber at an outlet endof the extended shallow portion.

In another step, foamy molten glass is introduced into the upstreamportion of the interior chamber through an inlet of the gradient finingtank. The foamy molten glass intermixes with and is subsumed by themolten glass bath that is maintained within the interior chamber of thegradient fining tank. In yet another step, the molten glass bath isdirected to flow along the extended shallow portion and into and alongthe deep holding portion in a flow direction through the interiorchamber. The molten glass bath has a zone of refined molten glass thatcollects adjacent to the outlet end of the deep holding portion abovethe exit section of the floor of the gradient fining tank. In stillanother step, refined molten glass is removed from the downstreamportion of the interior chamber through an outlet of the gradient finingtank. The outlet of the gradient fining tank is downwardly displacedfrom the inlet of the gradient fining tank and is open to a flow surfaceof the exit section of the floor.

According to yet another aspect of the present disclosure, a gradientfining tank includes a roof, a floor, and a pair of sidewalls thatconnect the roof and floor. The roof, floor, and sidewalls at leastpartially define an interior chamber for holding a molten glass bath.Moreover, the floor has an entry section that includes a flow surface,an exit section that includes a flow surface, and a transition sectionthat extends downwardly and outwardly from the entry section of thefloor to the exit section of the floor. The transition section of thefloor has a flow surface that slopes downwardly and outwardly from theflow surface of the entry section. The gradient fining tank alsoincludes an inlet that communicates with an upstream portion of theinterior chamber defined by an extended shallow portion of the gradientfining tank that includes the entry section of the floor. The extendedshallow portion of the gradient fining tank has an inlet end and anoutlet end. Additionally, the gradient fining tank includes an outletthat communicates with a downstream portion of the interior chamberdefined by a deep holding portion of the gradient fining tank thatincludes the transition section and the exit section of the floor. Thedeep holding portion of the gradient fining tank has an inlet end thatcommunicates with the outlet end of the extended shallow portion and anoutlet end, and a depth of the downstream portion of the interiorchamber at the outlet end of the deep holding portion is greater than adepth of the upstream portion of the interior chamber at the outlet endof the extended shallow portion. The outlet of the gradient fining tankis downwardly displaced from the inlet of the gradient fining tank.

BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure, together with additional objects, features, advantages,and aspects thereof, will be best understood from the followingdescription, the appended claims, and the accompanying drawings, inwhich:

FIG. 1 is an elevated partial cross-sectional view of a gradient finingtank according to one embodiment of the present disclosure;

FIG. 2 is a plan view of the gradient fining tank illustrated in FIG. 1;

FIG. 3 is an elevated partial cross-sectional view of a gradient finingtank according to another embodiment of the present disclosure; and

FIG. 4 is an elevated perspective view of a gradient fining tankaccording to various embodiments along with a submerged combustionmelter that is producing foamy molten glass for delivery to the gradientfining tank.

DETAILED DESCRIPTION

The present disclosure is directed to a gradient fining tank thatrefines foamy molten glass. The gradient fining tank is constructedspecifically to refine foamy molten glass that is produced in, anddelivered from, an upstream submerged combustion melter in a practicalmanner. The gradient fining tank relies primarily on gravity to collectrefined molten glass that has a satisfactory low concentration ofbubbles, seeds, and other inclusions as needed to facilitate downstreamprocessing into, for example, a glass container. As will be furtherdescribed below, a floor of the gradient fining tank is profiled toprovide the tank with an extended shallow portion and a deep holdingportion that allows a bath of molten glass maintained within an interiorchamber of the tank to naturally separate by density with the highestdensity glass accumulating at the bottom of the deep holding portionnear an outlet from interior chamber as refined molten glass. Therefining capability of the tank may be augmented with one or moreskimmers and/or one or more heat supply devices in an effort to speed upthe refining process and possibly allow for a smaller tank structure.Foamy molten glass may be introduced into the gradient fining tank andultimately refined with or without the use of chemical fining agents.

Referring now to FIG. 1, a first embodiment of the disclosed gradientfining tank is identified by reference numeral 10. The gradient finingtank 10 includes a roof 12, a floor 14, two laterally-spaced sidewalls16, 18 that connect the roof 12 and the floor 14, a front end wall 20,and a rear end wall 22. These wall structures 12, 14, 16, 18, 20, 22define an interior chamber 24 of the tank 10. The interior chamber 24and portions thereof are measurable using a coordinate system thatincludes three orthogonal axes-notably, a length dimension L, a widthdimension W, and a depth dimension D. The length dimension L is alongitudinal dimension that extends along a flow direction F through theinterior chamber 24, the width dimension W is a lateral dimension thatextends between the laterally-spaced sidewalls 16, 18 transverse to thelength dimension L, and the depth dimension D is a vertical dimensionthat extends between the roof 12 and the floor 14 orthogonal to theplane of the length and width dimensions L, W. The gradient fining tank10 also includes an inlet 26 to the interior chamber 24 and an outlet 28from the interior chamber 24. A flow 30 of foamy molten glass isintroduced into the interior chamber 24 through the inlet 26 and a flow32 of refined molten glass is discharged from the interior chamber 24through the outlet 28.

The floor 14 of the gradient fining tank 10 is profiled to provide thetank 10 with an extended shallow portion 10 a and a deep holding portion10 b positioned downstream of the extended shallow portion 10 a relativeto the flow direction F through the interior chamber 24. Specifically,the floor 14 includes an entry section 14 a, a transition section 14 b,and an exit section 14 c in continuous successive order along the flowdirection F. The entry section 14 a provides the extended shallowportion 10 a of the tank 10 along with the front end wall 20 andcorresponding portions of the roof 12 and the sidewalls 16, 18. Thetransition section 14 b and the exit section 14 c likewise provide thedeep holding portion 10 b of the tank 10 along with the rear end wall 22and corresponding portions of the roof 12 and sidewalls 16, 18. Each ofthe extended shallow portion 10 a and the deep holding portion 10 bdefines part of the interior chamber 24. In that regard, an upstreamportion 24 a of the interior chamber 24 is defined by the extendedshallow portion 10 a and a downstream portion 24 b of the interiorchamber 24 is defined by the deep holding portion 10 b.

The extended shallow portion 10 a of the tank 10 includes an inlet end34 and an outlet end 36. The inlet end 34 is located adjacent to thefront end wall 20. The inlet 26 of the tank 10 communicates with theupstream portion 24 a of the interior chamber 24 at the inlet end 34 ofthe extended shallow portion 10 a and, consequently, provides a conduitfor the flow 30 of foamy molten glass to enter into the upstream portion24 a of the interior chamber 24. The inlet 26 may be defined in the roof12, as shown, although it could also be defined in the front end wall 20or one or both of the sidewalls 16, 18. The outlet end 36 of theextended shallow portion 10 a is spaced apart from the inlet end 34 inthe flow direction F and communicates directly with the deep holdingportion 10 b of the tank 10. Within the extended shallow portion 10 a,the entry section 14 a of the floor 14 has flow surface 38 over whichmolten glass flows towards the deep holding portion 10 b. The flowsurface 38 of the entry section 14 a is preferably level or slightlydownwardly inclined relative to horizontal such that gravity supportsthe forward flow of molten glass.

The deep holding portion 10 b of the tank includes an inlet end 40 andan outlet end 42. The inlet end 40 communicates directly with the outletend 36 of the extended shallow portion 10 a. The outlet end 42 islocated adjacent to the rear end wall 22 and is spaced apart from theinlet end 40 in the flow direction F. The outlet 28 of the tank 10communicates with the downstream portion 24 b of the interior chamber 24at the outlet end 42 of the deep holding portion 10 b and, consequently,provides a conduit for the flow 32 of refined molten glass to exit thedownstream portion 24 b of the interior chamber 24. The outlet 28 may bedefined in the rear end wall 22, as shown, although it could also bedefined in the floor 14—more specifically, the exit section 14 c of thefloor 14—or one or both of the sidewalls 16, 18. Within the deep holdingportion 10 b, the transition section 14 b of the floor 14 extendsdownwardly and outwardly from the entry section 14 a of the floor 14 tothe exit section 14 c of the floor 14. Accordingly, the outlet 28 of thetank 10 is displaced downwardly (i.e., below in the depth dimension D)from the inlet 26 of the tank 10, and a depth d3 of the downstreamportion 24 b of the interior chamber 24 that is partially defined by theexit section 14 c of the floor 14 is greater at the outlet end 42 of thedeep holding portion 10 b than a depth d1 of the upstream portion 24 aof the interior chamber 24 at the outlet end 36 of the extended shallowportion 10 a.

The transition section 14 b in this particular embodiment includes aflow surface 44 that slopes downwardly and outwardly from the flowsurface 38 of the entry section 14 a of the floor 14 to a flow surface46 of the exit section 14 c of the floor 14. The flow surface 44 of thetransition section 14 b slopes downwardly from the flow surface 38 ofthe entry section 14 a of the floor 14 at an angle α that preferablyranges from 30° to 75° or, more narrowly from 45° to 60°, from ahorizontal imaginary plane 48 extending from a downstream end of theflow surface 38 of the entry section 14 a included in the extendedshallow portion 10 a. The horizontal imaginary plane 48 is level withrespect to gravity. As such, a depth d2 of the downstream portion 24 bof the interior chamber 24 progressively increases along the transitionsection 14 b of the floor 14 in the flow direction F. The flow surface46 of the exit section 14 c of the floor 14 extends outwardly from theflow surface 44 of the transition section 14 b at an angle β thatpreferably ranges from 115° to 160° or, more narrowly, from 130° to145°. The flow surface 46 of the exit section 14 c is preferably levelor slightly downwardly inclined (e.g, up to 10°) relative to horizontalsuch that gravity supports the forward flow of molten glass to theoutlet end 42 of the deep holding portion 10 b of the tank 10. Theoutlet 28 of the tank 10 is also preferably open to the flow surface 46of the exit section 14 c of the floor 14; that is, the flow surface 46of the exit section 14 c is flush with the outlet 28 such that the flow32 of refined molten glass is sourced from molten glass that lies alongthe flow surface 46 of the exit section 14 c.

The gradient fining tank 10 is sized so that the interior chamber 24 canhold a steady-state volume of a molten glass bath 50 that provides theresidence time needed for the flow 30 of foamy molten glass to beincorporated into and refined within the bath 50 before being dischargedas the flow 32 of refined molten glass. The size of the gradient finingtank 10 depends on a number of factors including the flow rate of theflow 30 of foamy molten glass and the residence time needed to attainquality refining. In many instances, the tank 10 is sized so that theinterior chamber 24 can hold anywhere between 1 and 50 metric tons ofmolten glass as the molten glass bath 50 with a surface level 52 of thebath 50 being high enough that the bath 50 partially fills both theupstream and downstream portions 24 a, 24 b of the interior chamber asshown in FIG. 1. In other words, the molten glass bath 50 is containedin both the extended shallow portion 10 a and the deep holding portion10 b of the tank 10 and contacts the front end wall 20, the rear endwall 22, both sidewalls 16, 18, and the floor 14, but not the roof 12.The surface level 52 of the molten glass bath 50 is constant in theupstream and downstream portions 24 a, 24 b of the interior chamber 24.

In one particular implementation of the gradient fining tank 10, alength 11 of the upstream portion 24 a of the interior chamber 24 withinthe extended shallow portion 10 a may range from 0.5 m to 5.0 m and thedepth d1 of the upstream portion 24 a of the interior chamber 24 mayrange from 0.2 m to 1.0 m at all points between the inlet end 34 and theoutlet end 36 of the extended shallow portion 10 a. Additionally, alength 13 of the downstream portion 24 b of the interior chamber 24 thatis partially defined by the exit section 14 c of the floor 14 may rangefrom 1.0 m to 5.0 m and the depth d3 of that same part of the downstreamportion 24 b may range from 1.0 m to 10.0 m at all points along the exitsection 14 c of the floor 14. Still further, a length 12 of thedownstream portion 24 b of the interior chamber 24 that is partiallydefined by the transition section 14 b of the floor 14 may range from1.0 m to 5.0 m and the depth d2 of that same part of the downstreamportion 24 b may range from 0.2 m to 10.0 m at all points along thetransition section 14 bc of the floor 14 with the understanding thatthis depth d2 progressively increases from the depth d1 of the upstreamportion 24 a of the interior chamber 24 at the outlet 36 of the extendedshallow portion 10 a and the depth d3 of the downstream portion 24 bdefined partially by the exit section 14 c of the floor 14 at theintersection of the transition and exit sections 14 b, 14 c of the floor14.

Each of the roof 12, the floor 14, the two laterally-spaced sidewalls16, 18, the front end wall 20, and the rear end wall 22 may beconstructed from a material that can adequately withstand the hightemperature and corrosive nature of molten glass such as, for example, arefractory material. A refractory material is a chemically resistantmaterial that has a high melting temperature among other relevantproperties including a low thermal conductivity and thermal shockresistance. A wide variety of refractory materials may be employed toconstruct the structural walls 12, 14, 16, 18, 20, 22 of the gradientfining tank 10. Refractory oxide materials including zirconia, fusedcast alumina-zirconia-silica (AZS), and alumina-silica are severalspecific refractories that may be used in the construction of the tank10, although they are certainly not the only viable candidates, ascertain refractory metals or refractory metal alloys, such as nickelalloys, could also be used in the construction of the tank 10. In someinstances, the floor 14 and the portions of the sidewalls 16, 18 and endwalls 20, 22 that are exposed to molten glass may be constructed fromone type of refractory material, while the roof 12 and the portions ofthe sidewalls 16, 18 and end walls 20, 22 that are not exposed to moltenglass may be constructed from another type of refractory material.

The gradient fining tank 10 may optionally include various types ofaugmentation equipment to improve the refining capability of the tank10, although such equipment is not necessarily required in order toattain quality refined glass within the tank 10. For instance, the tank10 may include one or more skimmers 54 that extend downwardly from theroof 12 into the molten glass bath 50 and laterally between thesidewalls 16, 18. In one particular embodiment, as shown here in FIG. 1,a skimmer 54 a may extend downwardly from the roof 12 so that a free end56 of the skimmer 54 a is submerged into the molten glass bath 50 withinthe deep holding portion 10 b of the tank 10 above the transitionsection 14 b of the floor 14. In this way, the free end 56 of theskimmer 54 a cooperates with the transition section 14 b of the floor 14and the adjoining portions of the sidewalls 16, 18 to define arestricted passage 58 that impedes the admittance of low density moltenglass into the deep holding portion 10 b downstream of the skimmer 54 ain the flow direction F. Additional skimmers may be located upstream ordownstream of the skimmer 54 a shown here in FIG. 1.

The gradient fining tank 10 may also include one or more heating devices60 that transfer heat to the molten glass bath 50 either alone or incombination with the one or more skimmers 54. If present, the one ormore heating devices 60 may include one or more impingement burners 60 amounted in the roof 12, the front end wall 20, and/or in one or both ofthe sidewalls 16, 18 above the surface level 52 of the molten glass bath50 of the tank 10 at least within the extended shallow portion 10 a.These burners 60 a burn a mixture of fuel and oxidant and direct theresultant flames 62 downward into contact with the surface level 52 ofthe molten glass bath 50 in the tank 10. By impinging the surface of themolten glass bath 50 with the emitted flames 62, the stable layer ofbubbles that tends to form on the surface of the bath 50 as part of alower-density molten glass phase of the bath 50 can be disrupted toallow gas bubbles to continue to burst and escape from the molten glassbath 50. One or more exhaust ducts 53 may communicate with the spaceabove the molten glass bath 50 through the roof 12, sidewalls 16, 18,and/or end walls 20, 22 to remove combustion products emitted from theimpingement burners 60 a or any other gaseous products that may need tobe vented from the gradient fining tank 10. If more than one exhaustduct 53 is present, the several exhaust ducts 53 may vent to a commonexhaust manifold 55, if desired.

Additionally, and as shown here, the tank 10 may further include one ormore non-impingement burners 60 b mounted in the roof 12, the rear endwall 22, and/or in one or both of the sidewalls 16, 18 above the surfacelevel 52 of the molten glass bath 50 at least within the deep holdingportion 10 b of the tank 10. The one or more non-impingement burners 60b burn a mixture of fuel and oxidant, although the flames 64 produced donot impinge the molten glass bath 50. Rather, the burners 60 b transferheat to the molten glass bath 50 by radiation from the space above themolten glass bath 50 to facilitate a faster rate of refining sincegaseous inclusions are more readily liberated as the temperature ofmolten glass increases and its viscosity decreases. If the skimmer 54 ais included in the structure of the tank 10, the impingement burners 60a are preferably located upstream of the skimmer 54 a, and thenon-impingement burners 60 b are preferably located downstream of theskimmer 54 a. Still further, the tank 10 may include electric heaters 60c installed within the floor 14 of the tank 10, if desired, in either orboth of the extended shallow portion 10 a and the deep holding portion10 b of the tank 10.

Referring now to FIG. 4, the gradient fining tank 10 is preferably usedto refine foamy molten glass produced by a submerged combustion (SC)melter 66. The SC melter 66 includes submerged burners 68—typically ofthe oxy-fuel type located in a floor 70 of the melter 66—thatindividually discharge a mixture of fuel and oxidant that autoigniteswithin a turbulent bath 72 of molten glass to cause vigorous agitationwithin the bath 72 as the combustion gasses 74 are thrust through thebath 72. These combustion gasses 74 eventually escape the molten glassbath 72 and are removed from the SC melter 66 through an exhaust duct76. In operation, a glass feedstock 78 is fed into the SC melter 66 andonto the molten glass bath 72 through a feeder 80. The vigorous stirringcaused by the submerged burners 68 creates intense heat transfer andrapid particle dissolution throughout the bath 72. This enables theglass feedstock 78 to mix, react, and become chemically integrated intothe molten glass bath 72 relatively quickly. A flow 82 of foamy moltenglass is directed from the molten glass bath 72 contained in the SCmelter 66 through a discharge conduit 84 located at the end of the SCmelter 66 where the molten glass is most chemically homogenized. Themolten glass in the outlet flow 82 is foamy in that it contains ahomogeneous distribution of entrained bubbles and has a density thatusually ranges from 0.99 gm/cm³ to 1.3 gm/cm³. The foamy nature of thisflow 82 of molten glass is attributable to the agitation and stirring ofthe molten glass bath 72 induced by the submerged burners 68. Typically,the gaseous inclusions entrained within the molten glass bath 72 vary insize and contain any of several gasses including CO, CO₂, H₂O (vapor),SO₂, CH₄, and volatile organic compounds (VOCs).

The glass feedstock 78 charged into the SC melter 66 is a material or amixture of materials from which the final glass chemistry can beobtained upon melting. For example, if soda-lime-silica glass is beingmade, the glass feedstock 78 may be a physical mixture of virgin rawmaterials and cullet (i.e., recycled glass) that provides a source ofSiO₂, Na₂O, and CaO in the correct proportions. The virgin raw materialsmay include corresponding quantities of quartz sand (i.e., crystallineSiO₂), soda ash, and limestone as the source of SiO₂, Na₂O, and CaO,respectively. Other virgin raw materials may also be included in theglass feedstock 78 to provide a source of one or more of SiO₂, Na₂O, CaOand possibly other oxide and/or non-oxide materials depending on thechemistry of the soda-lime-silica glass being produced. These othervirgin raw materials may include feldspar, dolomite, and calumite slag.Additionally, the glass feedstock 78 may include virgin raw materialsthat provide any of the network formers, network modifiers, colorants,decolorants, redox agents, or other agents listed above as well one ormore fining agents. The glass feedstock 78 may include up to 80 wt %cullet—with the remainder typically being entirely or mostly virgin rawmaterials—depending on a variety of factors including thecharacteristics (color, transparency, etc.) of the soda-lime-glass beingmanufactured. Other materials such as the glass precursor gel describedin US2016/0289114A1 may be included in the glass feedstock 78 inaddition to virgin raw materials and, if present, any cullet, and insome instances may even be used exclusively as the glass feedstock 78.

The flow 82 of foamy molten glass that exits the SC melter 66 throughthe discharge conduit 84 needs to be refined so that a finished glassarticle, such as a glass container, produced from the molten glass isfree of visual commercial variations. Glass refining is the process inwhich gaseous inclusions such as bubbles and seeds are removed from themolten glass. The resultant refined molten glass need not be perfectlyfree of gaseous inclusions to be commercially acceptable. In manyinstances, for example, refined molten glass can achieve a density thatranges from 2.3 gm/cm³ to 2.5 gm/cm³. The glass refining process here ismore complex and challenging compared to the refining of molten glassthat is produced in a conventional melting furnace that does not employsubmerged burners. This is because the flow 82 of foamy molten glassincludes a greater quantity of homogeneously distributed gaseousinclusions than the molten glass produced in a conventional furnace dueto the severe fluid disturbances caused by the submerged burners 68. Thegradient fining tank 10 is designed to refine this type of foamy moltenglass efficiently.

Glass refining using the gradient fining tank 10 involves, first,introducing the flow 30 of foamy molten glass into the interior tankchamber 24 through the inlet 26 of the tank 10. The flow 30 of foamymolten glass may be all or part of the flow 82 of foamy molten glassthat is discharged from the SC melter 66, or it may be a flow of foamymolten glass that is indirectly derived from the flow 82 of foamy moltenglass that is discharged from the SC melter 66 such as, for example,when the a stilling tank or holding tank is fluidly positioned betweenthe SC melter 66 and the fining tank 10. In one particular embodiment,as shown generally in FIG. 4, the flow 82 of foamy molten glassdischarged from the SC melter 66 is introduced directly into theinterior chamber 24 of the gradient fining tank 10 through the inlet 26as the flow 30 of foamy molten glass. The flow 30 of foamy molten glassmay fall from the discharge conduit 84 of the SC melter 66 under theforce of gravity through the inlet 26 of the gradient fining tank 10, asshown, or it may enter the interior chamber 24 in another way. Onceintroduced into the interior tank chamber 24 of the tank 10, the flow 30of foamy molten glass intermixes with, and is subsumed by, the moltenglass bath 50 contained within the interior chamber 24 and, morespecifically, within the upstream portion 24 a of the interior chamber24 defined by the extended shallow portion 10 a of the tank 10.

The molten glass bath 50 maintained within the interior chamber 24 flowsfrom the inlet 26 of the tank 10, which communicates with the upstreamportion 24 a of the interior chamber defined by the extended shallowportion 10 a, to the outlet 28 of the tank 10, which communicates withthe downstream portion 24 b of the interior chamber 24 defined by thedeep holding portion 10 b, along the flow direction F. The surface level52 of the molten glass bath 50 generally remains constant despite thismovement as the flow 32 of refined molten glass that is discharged fromthe interior chamber 24 through the outlet 28 of the tank 10 offsets theflow 30 of foamy molten glass being introduced into the interior chamber24. Consequently, as the flow 30 of foamy molten glass is introducedinto the interior chamber 24 through the inlet 26, the molten glass bath50 flows along the flow surface 38 of the entry section 14 a of thefloor 14 from the inlet end 34 to the outlet end 36 of the extendedshallow portion 10 a. During the time the molten glass bath 50 isflowing through the extended shallow portion 10 a, the molten glassachieves a density differential as the gaseous inclusions containedwithin the molten glass begin to rise. Usually, in the extended shallowportion 10 a of the tank 10, the molten glass bath 50 separates into alower-density phase 86 and a higher-density phase 88. The lower-densityphase 86 of the molten glass bath 50 rests on top of the higher-densityphase 88 and has a higher concentration of gaseous inclusions than thehigher-density phase 88. Typically, the lower-density phase 86 has adensity that ranges from 0.7 gm/cm³ to 1.0 g/cm³ while thehigher-density phase 88 has a density that ranges from above 1.0 g/cm³to 1.5 g/cm³.

The molten glass bath 50 continues to flow from the upstream portion 24a of the interior chamber 24 defined by the extended shallow portion 10a into the downstream portion 24 b of the interior chamber 24 defined bythe deep holding portion 10 b. More specifically, the molten glass bath50 flows directly into the inlet end 40 of the deep holding portion 10 bfrom the outlet end 36 of the extended shallow portion 10 a. Thedownward and outward extension of the transition section 14 b of thefloor 14, and thus the increasing depth of the downstream portion 24 bof the interior chamber 24, enables the heavier and denser molten glassof the higher-density phase 88 to sink and flow over the flow surface 44of the transition section 14 b and eventually over the flow surface 46of the exit section 14 c of the floor 14 within the deep holding portion10 b of the tank 10 in the flow direction F. The lower-density phase 86of the molten glass bath 50 either stays near the top of the bath 50 inthe deep holding portion 10 b or, if the skimmer 54 a is present, isblocked from flowing further downstream of the skimmer 54 a as only thehigher-density phase 88 of the bath 50 is permitted to pass through therestricted passage 58.

The density of the molten glass bath 50 thus increases along the floor14 of the tank 10 in the flow direction F as well as from the surfacelevel 52 of the bath 50 towards the floor 14 of the tank 10. Thisdensity profile is represented in a general manner by arrows D. Comparedto a tank that has a constant depth along its length, the density of themolten glass bath 50 increases more rapidly along the floor 14 of thegradient fining tank 10 in the flow direction F, mainly because of thechange in depth of the tank 10 engendered by the transition section 14 bof the floor 14, which means that the residence time needed tosatisfactorily refine the foamy molten glass introduced into the tank 10is shortened.

As a result of the profile of the floor 14 of the tank 10 and the flowof the molten glass bath 50, the density of the molten glass bath 50 isgreatest, and the concentration of entrained gaseous inclusions islowest, within the downstream portion 24 b of the interior chamber 24 atthe outlet end 42 of the deep holding portion 10 b of the tank 10. Tothat end, a zone 90 of refined molten glass collects within the moltenglass bath 50 adjacent to the outlet end 42 of the deep holding portion10 b and in fluid communication with the outlet 28 of the tank 10. Theflow 32 of refined molten glass that is discharged from the gradientfining tank 10 is supplied by the zone 90 of refined molten glasscontinuously. The flow 32 of refined molten glass, which has attained alevel of chemical and thermal homogeneity along with being adequatelyfree of gaseous inclusions, may then be transported away from the tank10 for further processing. For instance, in the manufacture of glasscontainers, the flow 32 of refined molten glass may be directed througha forehearth to a glass feeder. The forehearth is an extended channelthat functions to cool refined molten glass at a controlled rate to atemperature and viscosity suitable for glass forming operations whilealso achieving a more uniform temperature profile within the moltenglass. The glass feeder distributes runners of the molten glass to ashearing mechanism that shears the runners into glass gobs, whichsubsequently fall into a gob distribution system and are routed to blankmolds of individual sections of an individual section forming machine.

The gradient fining tank 10 shown in FIGS. 1-2 and describedspecifically above is subject to a variety of structural variationswithout necessarily compromising the goal of efficiently refining foamymolten glass. Referring now to FIG. 3, for example, a second embodimentof the disclosed gradient fining tank is identified by reference numeral110. In this embodiment, reference numerals that correspond to thepreviously-described gradient fining tank 10 are used to designatecorresponding structural features. As such, the use of a corresponding100 series reference numeral in connection with the gradient fining tank110 shown in FIG. 3 incorporates the earlier discussion of that samefeature in the gradient fining tank 10 shown in FIGS. 1-2 as if fullyreiterated here along with any additional modifying disclosure. Thefollowing discussion of the gradient fining tank 110 is thereforelimited to the material difference(s) relative to thepreviously-described gradient fining tank 10 with the understanding thatthe rest of the tank 110 is the same as before.

The gradient fining tank 110 shown here in FIG. 3 is essentially thesame as the previously-described gradient fining tank 10 except for theconstruction of the transition section 114 b of the floor 114. As shown,the transition section 114 b of the floor 114 still extends downwardlyand outwardly from the entry section 114 a of the floor 114 to the exitsection 114 c of the floor 114. But rather than having a flow surfacethat slopes downwardly from the flow surface 138 of the entry section114 a to the flow surface 146 of the exit section 114 c, the transitionsection 114 b of the floor 114 in this embodiment includes a step 92. Inparticular, the transition section 114 b includes an upper flow surface94 that slopes downwardly at the angle α (e.g., from 30° to 75° or, morenarrowly, from 45° to 60°) from the horizontal imaginary plane 148extending from a downstream end of the flow surface 138 of the entrysection 114 a included in the extended shallow portion 110, as before,and further includes an intermediate step flow surface 96 and a lowerflow surface 98. The intermediate step flow surface 96 extends outwardlyfrom the upper flow surface 94 at an angle γ that that preferably rangesfrom 115° to 160° or, more narrowly, from 130° to 145°, and the lowerflow surface 98 slopes downwardly at an angle θ that preferably rangesfrom 30° to 75° or, more narrowly from 45° to 60°, from a horizontalimaginary plane 99 extending from a downstream end of the intermediatestep flow surface 96 to the flow surface 146 of the exit section 114 cof the floor 114. Like the horizontal imaginary plane 148, thehorizontal imaginary plane 99 is level with respect to gravity. Theangle β shown here is the same as before. In this way, the depth d2 ofthe downstream portion 124 b of the interior chamber 124 increasesunevenly along the transition section 114 b of the floor 114 in the flowdirection F.

The transition section 114 b of the floor 114 in this embodiment directsthe flow of the heavier and denser molten glass in the higher-densityphase 188 of the molten glass bath 150 toward the outlet end 142 of thedeep holding portion 110 b along a slightly different course compared tothe previously-described gradient fining tank 10. The higher-densityphase 188 may assume a rippling current as it flows down along the flowsurfaces 94, 96, 98 of the transition section 114 b of the floor 114 upand over the step 92. Despite the change in flow behavior, the heavierand denser molten glass of the higher-density phase 188 still sinks andflows over the flow surfaces 94, 96, 98 of the transition section 114 band eventually over the flow surface 146 of the exit section 114 c ofthe floor 114 within the deep holding portion 110 b of the tank 110 inthe flow direction F. In that regard, and similar to the previousembodiment, the density of the molten glass bath 150 increases along thefloor 114 of the tank 110 in the flow direction F as well as from thesurface level 152 of the bath 150 towards the floor 114 of the tank 110,as is represented again in a general manner by arrows D.

There thus has been disclosed a gradient fining tank and a method ofrefining foamy molten glass within the tank that satisfies one or moreof the objects and aims previously set forth. The foamy molten glass maybe produced in an upstream melter and then provided either directly orindirectly to the gradient fining tank. The disclosure has beenpresented in conjunction with several illustrative embodiments, andadditional modifications and variations have been discussed. Othermodifications and variations readily will suggest themselves to personsof ordinary skill in the art in view of the foregoing discussion. Forexample, the subject matter of each of the embodiments is herebyincorporated by reference into each of the other embodiments, forexpedience. The disclosure is intended to embrace all such modificationsand variations as fall within the spirit and broad scope of the appendedclaims.

1. A method of refining foamy molten glass, the method comprising:introducing foamy molten glass into an interior chamber of a gradientfining tank defined at least in part by a roof, a floor, and a pair oflaterally-spaced sidewalls that connect the roof and the floor, thefoamy molten glass being introduced into the interior chamber through aninlet of the gradient fining tank that communicates with an upstreamportion of the interior chamber defined by an extended shallow portionof the tank, and, wherein, upon introduction into the upstream portionof the interior chamber, the foamy molten glass intermixes with a moltenglass bath that is maintained within the interior chamber of thegradient fining tank; directing the molten glass bath to flow along theextended shallow portion of the gradient fining tank from an inlet endof the extended shallow portion to an outlet end of the extended shallowportion, the extended shallow portion including an entry section of thefloor, and the molten glass bath separating into a higher-density phaseand a lower-density phase that rests on top of the higher-density phasewithin the extended shallow portion of the gradient fining tank;directing the molten glass bath to flow into a downstream portion of theinterior chamber defined by a deep holding portion of the gradientfining tank, the deep holding portion having an inlet end, whichcommunicates with the outlet end of the extended shallow portion, and anoutlet end, the deep holding portion further including a transitionsection of the floor that extends downwardly and outwardly from theentry section of the floor of the extended shallow portion to an exitsection of the floor such that a depth of the downstream portion of theinterior chamber at the outlet end of the deep holding portion isgreater than a depth of the upstream portion of the interior chamber atthe outlet end of the extended shallow portion; directing the moltenglass bath to flow along the deep holding portion from the inlet end ofthe deep holding portion to the outlet end of the deep holding portion,the higher-density phase of the molten glass bath sinking and collectingas refined molten glass within the downstream portion of the interiorchamber above the exit section of the floor at the outlet end of thedeep holding portion of the tank; and removing refined molten glass fromthe interior chamber of the gradient fining tank through an outlet ofthe gradient fining tank that communicates with the downstream portionof the interior chamber, the outlet of the gradient fining tank beingdownwardly displaced from the inlet of the gradient fining tank.
 2. Themethod set forth in claim 1, wherein the higher-density phase of themolten glass bath has a density that ranges from above 1.0 gm/cm³ to 1.5gm/cm³ and the lower-density phase of the molten glass bath has adensity that ranges from 0.7 gm/cm³ to 1.0 gm/cm³, within the extendedshallow portion of the gradient fining tank.
 3. The method set forth inclaim 1, wherein the entry section of the floor has a flow surface, andthe transition section of the floor has a flow surface that slopesdownwardly from the flow surface of the entry section of the floor at anangle that ranges from 30° to 75° from a horizontal imaginary planeextending from a downstream end of the flow surface of the entrysection.
 4. The method set forth in claim 3, wherein the flow surface ofthe transition section slopes downwardly to a flow surface of the exitsection such that a depth of the downstream portion of the interiorchamber progressively increases along the transition section, andwherein the flow surface of the exit section of the floor extendsoutwardly from the flow surface of the transition section of the floorat an angle that ranges from 115° to 160°.
 5. The method set forth inclaim 3, wherein the flow surface of the transition section of the flooris an upper flow surface that slopes downwardly to an intermediate stepflow surface of the transition section, the intermediate step flowsurface extending outwardly from the upper flow surface at an angle thatranges from 115° to 160°, and wherein the transition section furtherincludes a lower flow surface that slopes downwardly from theintermediate step flow surface at an angle that ranges from 30° to 75°from a horizontal imaginary plane extending from a downstream end of theintermediate step flow surface.
 6. The method set forth in claim 5,wherein the lower flow surface of the transition section of the floorslopes downwardly to a flow surface of the exit section of the floor,and wherein the flow surface of the exit section of the floor extendsoutwardly from the lower flow surface of the transition section of thefloor at an angle that ranges from 115° to 160°.
 7. The method set forthin claim 1, wherein the exit section of the floor has a flow surface,and the outlet of the gradient fining tank is open to the flow surfaceof the exit section.
 8. The method set forth in claim 1, wherein thedepth of the upstream portion of the interior chamber at the outlet endof the extended shallow portion of the gradient fining tank ranges from0.2 m to 1.0 m, and wherein the depth of the downstream portion of theinterior chamber at the outlet end of the deep holding portion of thegradient fining tank ranges from 1.0 m to 10 m.
 9. The method set forthin claim 1, wherein the foamy molten glass that is introduced into thegradient fining tank has a density that ranges from 0.9 gm/cm³ to 1.3gm/cm³, and wherein the refined molten glass that is removed from thegradient fining tank has a density that ranges from 2.3 gm/cm³ to 2.5gm/cm³.
 10. The method set forth in claim 1, wherein one or moreskimmers extend downwardly from the roof of the gradient fining tank andare submerged into the molten glass bath.
 11. The method set forth inclaim 10, wherein the one or more skimmers includes a skimmer thatextends downwardly from the roof of the gradient fining tank so that afree end of the skimmer is submerged into the molten glass bath abovethe transition section of the floor, the free end of the skimmercooperating with the transition section of the floor and the sidewallsto define a restricted passage that impedes the admittance of thelower-density phase of the molten glass bath downstream of the skimmer.12. The method set forth in claim 1, wherein the gradient fining tankincludes one or more heating devices that transfer heat to the moltenglass bath.
 13. The method set forth in claim 12, wherein the one ormore heating devices includes one or more impingement burners mounted inthe roof of the gradient fining tank within the extended shallow portionof the tank, the one or more impingement burners directing flamesdownwardly into contact with a top surface of the molten glass bath. 14.The method set forth in claim 12, wherein the one or more heatingdevices includes one or more impingement burners mounted in one or bothof the sidewalls above the molten glass bath within the extended shallowportion of the tank, the one or more impingement burners directingflames downwardly into contact with a top surface of the molten glassbath.
 15. A method of refining foamy molten glass, the methodcomprising: providing a gradient fining tank that includes a roof, afloor, and a pair of sidewalls that connect the roof and floor, theroof, floor, and sidewalls at least partially defining an interiorchamber that holds a molten glass bath, the gradient fining tank havingan extended shallow portion that defines an upstream portion of theinterior chamber and a deep holding portion that defines a downstreamportion of the interior chamber, wherein the extended shallow portion ofthe tank includes an entry section of the floor and the downstreamportion of the tank includes a transition section of the floor thatextends downwardly and outwardly from the entry section of the floor toan exit section of the floor such that a depth of the downstream portionof the interior chamber at an outlet end of the deep holding portion isgreater than a depth of the upstream portion of the interior chamber atan outlet end of the extended shallow portion; introducing foamy moltenglass into the upstream portion of the interior chamber through an inletof the gradient fining tank, the foamy molten glass intermixing with themolten glass bath that is maintained within the interior chamber of thegradient fining tank; directing the molten glass bath to flow along theextended shallow portion and into and along the deep holding portion ina flow direction through the interior chamber, the molten glass bathhaving a zone of refined molten glass that collects adjacent to theoutlet end of the deep holding portion above the exit section of thefloor of the gradient fining tank; and removing refined molten glassfrom the downstream portion of the interior chamber through an outlet ofthe gradient fining tank, the outlet of the gradient fining tank beingdownwardly displaced from the inlet of the gradient fining tank and opento a flow surface of the exit section of the floor.
 16. The method setforth in claim 15, further comprising directing the molten glass bath toflow through a restricted passage defined by the transition section ofthe floor, the sidewalls, and a free end of a skimmer that extendsdownwardly from the roof the gradient fining tank into the molten glassbath.
 17. The method set forth in claim 15, further comprisingintroducing heat to the molten glass bath from one or more heatingdevices selected from the group consisting of impingement burners,non-impingement burners, and electric heaters.
 18. The method set forthin claim 15, wherein the entry section of the floor has a flow surface,and the transition section of the floor has a flow surface that slopesdownwardly from the flow surface of the entry section of the floor at anangle that ranges from 30° to 75° from a horizontal imaginary planeextending from a downstream end of the flow surface of the entrysection, the flow surface of the transition section of the floor slopingdownwardly to a flow surface of the exit section of the floor such thata depth of the downstream portion of the interior chamber progressivelyincreases along the transition section, and wherein the flow surface ofthe exit section of the floor extends outwardly from the flow surface ofthe transition section of the floor at an angle that ranges from 115° to160°.
 19. The method set forth in claim 15, wherein the entry section ofthe floor has a flow surface, and the transition section of the floorhas an upper flow surface that slopes downwardly from the flow surfaceof the entry section of the floor at an angle that ranges from 30° to75° from a horizontal imaginary plane extending from a downstream end ofthe flow surface of the entry section, the transition section of thefloor further including an intermediate step flow surface that extendsoutwardly from the upper flow surface at an angle that ranges from 115°to 160°, and a lower flow surface that slopes downwardly from theintermediate step flow surface at an angle that ranges from 30° to 75°from a horizontal imaginary plane extending from a downstream end of theintermediate step flow surface, the lower flow surface of the transitionsection of the floor sloping downwardly to a flow surface of the exitsection of the floor, the flow surface of the exit section of the floorextending outwardly from the lower flow surface of the transitionsection of the floor at an angle that ranges from 115° to 160°.
 20. Agradient fining tank for refining foamy molten glass, the gradientfining tank comprising: a roof, a floor, and a pair of sidewalls thatconnect the roof and floor, the roof, floor, and sidewalls at leastpartially defining an interior chamber for holding a molten glass bath,the floor having an entry section that includes a flow surface, an exitsection that includes a flow surface, and a transition section thatextends downwardly and outwardly from the entry section of the floor tothe exit section of the floor, the transition section of the floorhaving a flow surface that slopes downwardly and outwardly from the flowsurface of the entry section of the floor; an inlet that communicateswith an upstream portion of the interior chamber defined by an extendedshallow portion of the gradient fining tank that includes the entrysection of the floor, the extended shallow portion of the gradientfining tank having an inlet end and an outlet end; and an outlet thatcommunicates with a downstream portion of the interior chamber definedby a deep holding portion of the gradient fining tank that includes thetransition section and the exit section of the floor, the deep holdingportion of the gradient fining tank having an inlet end thatcommunicates with the outlet end of the extended shallow portion and anoutlet end, wherein a depth of the downstream portion of the interiorchamber at the outlet end of the deep holding portion is greater than adepth of the upstream portion of the interior chamber at the outlet endof the extended shallow portion, and wherein the outlet of the gradientfining tank is downwardly displaced from the inlet of the gradientfining tank.
 21. The gradient fining tank set forth in claim 20, whereinthe flow surface of the transition section slopes downwardly to the flowsurface of the exit section such that a depth of the downstream portionof the interior chamber progressively increases along the transitionsection, or wherein the flow surface of the transition section of thefloor is an upper flow surface that slopes downwardly to an intermediatestep flow surface of the transition section that extends outwardly fromthe upper flow surface.